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Towards a Food Web Perspective on Biodiversity and Ecosystem Functioning

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Towards a Food Web Perspective on Biodiversity and Ecosystem Functioning CHAPTER 8 Towards a food web perspective on biodiversity and ecosystem functioning Bradley Cardinale, Emmett Duffy, Diane Srivastava, Michel Loreau, Matt Thomas, and Mark Emmerson 8.1 Introduction that they exert a disproportionate influence over One of the most common questions asked by food web dynamics. This idea has fueled much researchers across a variety of scientificdisciplinesis debate over the prevalence of omnivory in food ‘How does the number of nodes connected together webs (Polis and Strong 1996, Thompson et al. 2007, into a network influence the efficiency and reliability Yodzis 1984) and whether the increased number of of that network?’. While social scientists and epide- feeding links that result from omnivory increases or miologists might think of ‘nodes’ and ‘connections’ decreases the stability of energy flow through a as people interacting within a social network, com- food web (McCann et al. 1998, MacArthur 1955). puter scientists, neurologists, and civil engineers Identifying species that represent influential nodes would instead think of servers connected together in has also been one of the primary goals in the search a world-wide web, synapses connecting neurons in for ‘ecosystem engineers’ (Jones et al. 1994), ‘key- the brain, or hubs connecting to other hubs in a stone species’ (Paine 1966, Power et al. 1996) or transportation network or telecommunications grid other types of ‘strong interactors’ (Wootton and (Albert and Barabasi 2002, Newman 2003). Regard- Emmerson 2005) that might have cascading effects less of the particular study system, all of these on the diversity and biomass of species at a variety individuals ask similar questions about how the of different trophic levels (Paine 1966, Carpenter number of nodes and connections among nodes et al. 1987, Elser et al. 1988). influence the efficiency and reliability by which In the 1990s, ecologists began to pursue a slightly information, disease, energy, or matter is transmitted different perspective on food webs. This perspec- throughout that network. tive focused not on the cascading impacts of indi- Within the field of ecology, one of the oldest and vidual species, but rather on how the number of most fundamental questions asked by researchers is species that comprise any single trophic level might ‘How does the number of species interacting within control fluxes of energy and matter. Research in this a food web influence the efficiency and reliability area was generally referred to as Biodiversity effects by which energy and matter are transmitted on Ecosystem Functioning (BEF for short), and was through that web?’. Research on this topic can be often justified on grounds that (1) loss of biological broadly divided into two foci. Historically, much diversity ranks among the most pronounced chan- attention in ecology has focused on identifying ges to the global environment (Sala et al. 2000, those taxa that are the most influential nodes in a Pimm et al. 1995), and (2) reductions in diversity, food web. For many years, it has been thought that and corresponding changes in species composition, some subset of species might represent ‘hubs’ of may alter fluxes of energy and matter that underlie interactions and/or exhibit such strong interactions important services that ecosystems provide to 105 106 BIODIVERSITY, ECOSYSTEM FUNCTIONING, AND HUMAN WELLBEING (a) 25 100 (b) 20 80 15 60 10 40 % Studies Number species 5 20 0 0 123456 Trophic level Figure 8.1 (a) Summary of the biological complexity of biodiversity-ecosystem functioning (BEF) studies performed to date. On the x-axisisthe number of trophic levels included in different experiments. On the left-hand y-axis (plotted as grey bars) is the mean number of species per trophic level. On the right hand y-axis (plotted as triangles) is the percentage of studies that have included 1, 2, or more trophic levels. Note that 93 per cent of BEF experiments have focused on a single trophic level composed of a mean seven species. (b) An example of the complexity of a real, yet still relatively simple natural food web in a salt marsh (from Lafferety et al. 2007). Note that within this system there are dozens of species (nodes) and hundreds of feeding links (lines connecting nodes) among plants, herbivores, predators and parasites that span six or more trophic levels. Figure reproduced with permission from K. Lafferty. humanity (e.g. production of food, pest/disease simplifications are justified, or alternatively, whether control, water purification, etc. Daily 1997, Chapin et they have led ecologists to potentially erroneous al. 1998). While the value of BEF research for conser- conclusions. However, what is clear is that a large vation biology and management has been questioned body of research in ecology has shown that interac- by some (Schwartz et al. 2000, Srivastava and Vellend tions of species across trophic levels can have cas- 2005), there is a more fundamental reason for the cading impacts that influence the diversity and recent prominence of this topic. BEF is one of the few biomass of organisms at numerous levels in a food research topics in ecology that examines how bio- web. At the very least, this suggests that the past focus logical variation per se acts as an independent variable of BEF on diversity within single trophic levels may to regulate key community and ecosystem-level pro- be insufficient to quantitatively predict, and perhaps cesses (Naeem 2002b). Understanding the ecological even qualitatively reflect, the ecological consequences consequences of variation among species has shown of diversity loss. much potential to complement our historical focus on In this chapter, we continue with the development the ecological impacts of highly influential species. of an idea that originated with other authors who Although the BEF paradigm has evolved consid- have argued that, in order to understand how erably over the past 15–20 years and been increasingly extinction alters the functioning of whole ecosys- applied to a variety of organisms and ecosystems, tems, ecologists will likely need to merge modern studies have continued to focus mostly on simplified paradigms of BEF with much more classic ideas in ‘model’ communities. In fact, the typical experiment food web ecology that consider not only the func- has manipulated an average of just seven species in an tional role of diversity within trophic levels, but the average of just one trophic group (Fig. 8.1(a)). Such interactions of species across trophic levels (Duffy minimal levels of complexity are far from the realities et al. 2007, Bruno and Cardinale 2008, Petchey et al. of natural food webs, where, even for some of the 2004a). Our chapter is organized as follows. In Sec- simplest communities, species interact within webs tion 8.2 we brieflyreviewfive hypotheses about how composed of hundreds of species spanning many fluxes of energy and matter through a food web trophic levels (Lafferty et al. 2006, Polis 1991, Martinez might depend on the diversity of species comprising 1992). At present, it is unclear whether such over- a web. Those hypotheses are divided into those that TOWARDS A FOOD WEB PERSPECTIVE ON BIODIVERSITY AND ECOSYSTEM FUNCTIONING 107 contrast diversity effects within different trophic impacts on processes that are disproportionate to levels versus those that focus on diversity effects their abundance or biomass. Duffy’s(2002)paper across trophic levels. In Section 8.3 we outline the was one of the firsttocallforamergerofBEFand empirical support for or against these hypotheses, food-web theory, and the hypotheses put forth in emphasizing that most are still unresolved and in that paper were useful, in part, because they need of testing. In the final Section 8.4, we outline represented an alternative to those posed by a just a few of the areas of research that we believe number of other authors. For example, some have will be fruitful as ecologists move towards an inte- argued that extinction at higher trophic levels gration of BEF into food-web ecology. may, in fact, have less impact on ecological pro- cesses than extinction at lower trophic levels. These arguments have usually been based on the 8.2 Five early hypotheses about idea that animals are more generalized in their use multi-trophic biodiversity and ecosystem of resources than historically appreciated, either function because the extent of omnivory and intra-guild predation has been underestimated (Rosenheim et 8.2.1 Diversity effects within trophic levels al. 1995, Holt and Polis 1997, Polis and Holt 1992), 8.2.1.1 Top-down effects of diversity grow increasingly or because animals can ‘switch’ among different strong at higher trophic levels prey species by moving across habitats (Polis et al. 1997, McCann et al. 2005). Resource generalization has been proposed to dampen the effects of con- C1 Cn sumer diversity on prey populations (Finke and +++ Denno 2005, Snyder and Ives 2003). H1 Hn 8.2.1.2 Increasing diversity of a resource reduces the strength of top-down control by consumers ++ P P 1 n C + – N R1 Rn Early hypotheses proposed that species extinction from higher trophic levels was likely to have greater The majority of BEF studies performed to date have impacts on the functioning of ecosystems than taken a ‘top-down’ perspective, meaning that they extinction from lower trophic levels (Table 8.1). have examined how diversity within a given Duffy (2002) argued that three characteristics trophic level impacts the fraction of resources con- potentially make ecological processes more sensitive sumed, and production of biomass, by that focal to extinction by consumers than plants: (1) because trophic level. In contrast, diversity may also have species at higher trophic levels have lower popula- ‘bottom-up’ effects on the dynamics of food webs, tion sizes and are under stronger anthropogenic meaning that the diversity of resources may influ- pressure than most wild plants, higher trophic levels ence how efficiently those resources are consumed face greater risks of extinction and higher rates of and converted into biomass by higher trophic levels species loss; (2) consumer assemblages have lower (Table 8.1).
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